Weill said he hasn't spoken with Citigroup CEO Vikram Pandit, 55, or JPMorgan's Jamie Dimon, 56, about his change of heart. Dimon is a former protege of Weill's and helped build Travelers before the merger with Citicorp.

Wall Street chiefs have resisted calls to break up their companies. Morgan Stanley CEO James Gorman, 54, described the debate as a "knee-jerk discussion" in a June 27 interview.

Dimon's View

Dimon said he disagreed with a shareholder who asked on a July 13 conference call whether the bank had become too big to manage.

"I beg to differ," Dimon said. "There is huge strength in this company that the units get from each other."

Breaking up the banks into different parts would make the firms much more valuable, Weill said. The stocks of five of the six biggest U.S. banks -- Citigroup, JPMorgan, Bank of America, Goldman Sachs and Morgan Stanley -- are languishing at or below tangible book value. That means different pieces of the banks are worth more than the whole, fund manager Michael F. Price said last month.

Citigroup's shares trade at 50 percent of tangible book value and New York-based Morgan Stanley's are at 47 percent, according to data compiled by Bloomberg.

"Now you have the preeminent creator of the large financial-conglomerate model agreeing that large banks should be broken up," Michael Mayo, an analyst at CLSA Ltd. in New York who has covered the largest U.S. banks since before Glass- Steagall's repeal, said in an interview. "It's going to make some people pretty upset, since he's the one who created the current Citigroup model, and now he's saying, 'Look, we messed up.'"

Greenspan Speech

Even Alan Greenspan, who fought for the repeal of Glass-Steagall when he was chairman of the Federal Reserve, said in 2009 that breaking up the banks might make them more valuable.

"In 1911, we broke up Standard Oil -- so what happened?" Greenspan said at New York's Council on Foreign Relations. "The individual parts became more valuable than the whole. Maybe that's what we need to do."

Weill altered his view about the industry because "the world changes," he said, adding that he's "been thinking about it a lot over the last year."